Age-related changes of phoria myopia in patients with intermittent exotropia

Purpose  To investigate the age-related changes in a myopic shift under binocular conditions (phoria myopia) in patients with intermittent exotropia (IXT). Methods  Forty-five patients with IXT were studied: 21 were ≤9 years old (children), 11 were between 10 and 19 years (adolescents), and 13 were between 20 and 43 years (adults). The angle of strabismus was determined by the alternating prism cover test. The spherical refractive error was measured at 1 m using infrared video retinoscopy under monocular and binocular viewing conditions. Results  The change in the spherical refractive error (ΔR) between binocular and monocular conditions was significantly larger in adults (ΔR = −1.11 ± 1.01 diopters (D), average ± standard deviation) than in children (ΔR = −0.34 ± 0.34 D; P < 0.05, analysis of variance). ΔR was significantly correlated with the angle of exotropia only in adults (r = 0.55, P = 0.04). After strabismus surgery, ΔR decreased in adults (n = 3). Conclusions  Because a significant myopic shift under binocular conditions was detected in IXT patients older than 20 years, phoria myopia can occur after age 20 even if functional disturbances are not observed in children or adolescent IXT patients, a fact that specialists need to bear in mind when treating younger patients.